Coating composition

ABSTRACT

A coating composition based upon an aqueous acid binder solution containing phosphate and chromate and/or molybdate ions into which has been dispersed a blend of metallic powder particles of three primary dimensions and flat metal flake materials, which can be applied to a substrate part to be coated more uniformly and thickly than conventional metal flake coatings, and which imparts better corrosion and impact resistance than conventional powder metal coatings.

This application is a continuation-in-part, continuation, division, of application Ser. No. 006,376, filed 1/23/87 now abandoned.

This patent application incorporates in its entirety, the Disclosure Document Deposit (including all Figures) deposited in the United States Patent Office on Apr. 22, 1985, under No. 136,989 and it is made part hereof.

The present invention relates to coating compositions used to impart corrosion resistance to a variety of metallic articles. The coatings of the invention are also applicable to a variety of non-metals including ceramics and composite materials to provide surfaces suitable for bonding and surfaces with other modified properties.

The invention broadly described relates to aqueous compositions which comprise an aqueous acid binder which includes phosphate ions and chromate and/or molybdate ions and a powder mixture of flakes of a pigment having two primary dimensions and of a material which is not a flake, generally a metal which is three-dimensional in its primary dimension,

The invention also provides for the coatings of such compositions and the articles coated with the composition.

The coatings and coated articles have improved properties over the conventionally coated articles.

The invention also relates to the articles coated with such compositions and to a method for coating various articles. More particularly, the invention relates to aqueous acid coating compositions containing a dissolved phosphate compound and chromate or molybdate compound and particles of flake shaped pigment along a spherical, atomized or other three-dimensional pigment.

The invention is further illustrated by the following Figures, wherein

FIG. 1 is a graph showing the effect of pigment concentration on the viscosity of an acid binder in which the added pigments are aluminum flakes and atomized powders of different sizes and types.

FIG. 2 is a graph showing the effect of pigment concentration on the viscosity of an acid binder in which the axis has been relabeled to show the water content of the acid binder. The added pigments, aluminum flakes and atomized powders, are identical to those shown in FIG. 1.

FIG. 3 is a graph showing the effect of pigment type on the viscosity of an acid binder in which total pigment concentration is held constant and flake pigment is substituted for atomized powder.

FIG. 4 is a micrograph of a coating constituted by a phosphate/chromate binder and spherical aluminum powder but no flake powder.

FIG. 5 is a micrograph of a coating constituted by a phosphate/chromate binder and spherical aluminum powder which contains 6 weight percent flake aluminum.

FIG. 6 is a micrograph of a coating constituted by a phosphate/chromate binder and spherical aluminum powder which contains 12.5 weight percent flake aluminum.

FIG. 7 is a micrograph of a coating constituted by a phosphate/chromate binder and spherical aluminum powder which contains 19 weight percent flake aluminum.

FIG. 8 is a micrograph of a coating constituted by a phosphate/chromate binder and spherical aluminum powder which contains 25 weight percent flake aluminum.

Coating compositions are known which comprise a slurry of solid particulate material in an aqueous acidic solution containing phosphates and chromates, or molybdates. After application of such a coating to a substrate, it is heated to a temperature of about 500° F. until the coating is water insoluble. Higher or lower temperatures are often used. Such coatings often incorporate metal powders often referred to as "pigments", such as aluminum powder, as a solid particulate material so as to provide oxidation and corrosion protection to ferrous alloys. In some cases where the substrate for the coating is ferrous metal, the aluminum powder in the coating, by way of electrochemical reactions which ensue in a salt-spray or other corrosive atmosphere, generally sacrifices itself to the end that there is little or no corrosion of the ferrous metal substrate. It has further been established that the property of the sacrificiality of the coating in providing the increased corrosion protection is greatly enhanced by so processing the coating as to render it electrically conductive. Usually, the aluminum filled coatings are made sacrificial and galvanically active by either cold working (burnishing) the coating surface, or by heat treating to a temperature of about 1,000° F. for ninety minutes. Other heat treatments can be used. The goal is to produce an electrically conductive aluminum coating that is sacrificial, i.e., will corrode and protect the adjacent base metal areas from corrosion.

Sacrificial, burnished or heat treated coatings of this type are commonly used on aerospace and industrial turbine components. In other application, burnished or heat treated coatings may be used in salt environments and/or in elevated temperatures. Examples of this type of chromate/phosphate coating are described in the patent art. One early U.S. Patent, Allen 3,248,251, describes heat cured dispersions comprising a solid particulate material in an aqueous solution containing substantial amounts of phosphate anion, and an anion of chromate, molybdate, and mixtures thereof, preferably all chromate, and metal ion. Specific preferred solid particulate material is listed in column 8, lines 19-44. Of preference is metal powder, aluminum powder of spherical shape and 5-10 microns average diameter being most preferred. In related disclosures, Collins uses the same pigment in aluminum filled chromate/phosphate dispersions described in U.S. Pat. Nos. 3,248,249 and 3,248,250, the improvements being methods to reduce the cure temperature of the coating.

In Brumbaugh, U.S. Pat. No. 3,869,293, use of aluminum/magnesium alloys is disclosed as a means of improving corrosion resistance. Specifically the aluminum/magnesium alloy particles and also the aluminum particles present should preferably be of generally spherical shape often referred to as "atomized powder" (column 3, lines 5-8).

In U.S. Pat. No. 4,319,924 and its divisional case 4,381,323, Collins, Lowe and Klotz disclose similar compositions, but with the improvement of a lower temperature cure effected by additions of diethanolamine and the use of dissolved aluminum as part of the chromate/phosphate solution. The '924 patent suggests ways of selecting pigment material by size; particle shape is not disclosed, but use of terminology such as "aluminum metal powder, -400 mesh" indicates that atomized powder is being used in the compositions.

In U.S. Pat. No. 4,537,632, Mosser discloses the use of inert gas atomized spherical powder in chromate/phosphate coating compositions. The patent shows that improved corrosion resistance and erosion properties are imparted to coating films by selecting as a pigment spherical aluminum powder having an average equivalent spherical diameter of not larger than 4 microns.

Use of atomized aluminum powder is described in U.S. Pat. No. 4,544,408, Mosser and Eddinger, which also includes a monoprotic acid and bohemite alumina to produce thixotropic chromate/phosphate coatings.

Similar atomized aluminum powder is used in U.S. Pat. No. 4,548,646, Mosser and Fabiny, which also includes certain surface active agents and silica to produce thixotropic chromate/phosphate coatings. These coatings specifically call out atomized aluminum powder preferably less than 10 microns in diameter.

Other disclosures of Mosser and McMordie, such as Patent 4,617,056 and co-pending application 802,827, Parts Coated with Thick Coating Composition of Uni- and Polymodal Types, now U.S. Pat. No. 4,659,613 disclose the use of mixtures of large and small atomized aluminum powder having specific sizes and ratios of large to small particles. Such compositions have numerous uses such as corrosion resistant thick films and abradable seal materials.

All the aforementioned patents and disclosures disclosed the use of atomized aluminum powder, with various sizes being preferred in different applications.

In U.S. Pat. No. 3,318,716, Schuster and Baldi disclose a coating composition consisting of a leafing aluminum pigment, a liquid ethylene glycol and a specific quantity of wetting agent, said coating being heat cured to reduce much of the hexavalent chromium to trivalent chromium.

In U.S. Pat. No. 3,958,046, Baldi discloses substituting the leafing aluminum, along with sufficient wetting agent and with or without polymeric ethylene glycol, for the spherical aluminum in the formulations described in the Allens '251 patent. This can be seen in '046 column 6, line 25 to column 7, line 7. Baldi claims that the use of flake pigment as a substitute for spherical aluminum pigment contributes a significant increase in corrosion resistance, particularly in cured layers weighing not more than about 1 milligram per square centimeter. The coating is recommended for application over low temperature pack aluminide coatings out of contact with the work pieces.

In U.S. Pat. No. 4,141,760, Baldi discloses the use of the same type of flake-containing compositions as a topcoat over chromate/phosphate compositions containing atomized aluminum powder. This is described in column 10, lines 17-50. In addition, aluminum flake compositions are disclosed which may additionally contain polytetrafluoroethylene.

In U.S. Pat. No. 4,350,719, Baldi discloses the use of layers of flake aluminum coatings as described in the '760 patent used along with coating layers containing colloidal alumina, or layers containing flake aluminum with magnesium dichromate. Combinations of these layers are described as being utilized as multilayer coating systems for improved corrosion resistance and improved heat resistance.

In U.S. Pat. No. 3,671,331, Malkin et al disclose the use of a pulverulent metal in an acidic chromate solution. The pulverulent metal may be flake or powder or both and may include magnesium, aluminum, manganese and zinc and alloys of these metals. In addition, a chromate reducing agent is part of the composition. A larger amount of dispersing agent can be used. Phosphoric acid and phosphates can be present in very minor amounts (column 3, lines 43-56).

Other United States patents which have been noted in a search include the following:

No. 3,779,804 to Urban discloses a process for electroding ceramic semiconductors by means of application of compositions comprising fine aluminum powder, aluminum or magnesium phosphate and chromic acid followed by baking the coated articles at elevated temperature and finally applying silver paste to the coated surface.

No. 4,487,815 to Dorsett et al discloses coated substrates having two adjacent layers one containing a hexavalent chromium component, particulate zinc plus particulate aluminum and a boric acid component and the other layer being a silica layer substantially metal free.

No. 3,754,942 to Moradian discloses a protective coating for metals comprising a micro cystalline wax, paraffinic and naphthenic oils, asphaltite, naphthenic acid, an alkali metal sulfonate, a drying oil and optional aluminum paste as pigment.

No. 3,713,904 to Bernath et al discloses a resin containing composition that also utilizes phosphoric acid, strontium chromate and another chromate source. All hexavalent chromium in the composition is chemically reduced to trivalent chromium; the trivalent chromium being reoxidized to hexavalent chromium during the curing process. The composition does not include metal powder pigments.

In U.S. Pat. No. 4,238,534 to Inchida, a coating for steel is disclosed that comprises a monobasic phosphate, a dichromate source and an oxide pigment selected from SiO₂, Al₂ O₃ and TiO₂ along with a colloidal silica binder. The disclosed composition is heat resistant and is highly adherent to oriented silicon steel sheet.

In U.S. Pat. No. 3,687,739, Kennedy et al disclose a composite coating constituted of an undercoating and a top coating. The undercoating is constituted of a liquid containing a hexavalent-chromium (expressed as CrO₃) and a pulverulent metal which can be flake or powder.

No. 3,897,222 to Hood discloses a method of protecting ferrous metal articles, particularly steel bolts from corrosion by coating the fasteners with a thin film of a noble metal, such as nickel plate, followed by a second sacrificial aluminum filled layer specifically compositions described by Allen in U.S. Pat. No. 3,248,251.

No. 3,996,073 to Evans discloses insulative coatings for steel and are useful for improvements of the magnetic characteristics of the steels. The compositions comprise certain aluminum-magnesium phosphate ratios in a colloidal silica in water. Chromic acid can be included. Metal pigments are not disclosed.

A review of the aforementioned patents indicates that the majority of chromate/phosphate based compositions that utilize metallic pigmentation use atomized powder of various sizes for coatings with different properties. The references to flake coatings include in the compositions only flake material (as pigment). In certain references, both flake and atomized (or zinc dust) pigments were mixed together. The disclosed coatings were not chromate/phosphate compositions, but chromate only compositions using zinc and aluminum pigments. Use of zinc pigments is not contemplated in this invention due to high reactivity with the chromate/phosphate binder; and phosphate is an essential ingredient of the aqueous composition.

Metal powders are produced commercially in a variety of shapes and sizes. The specific manufacturing technique determines size range, distribution, shape and surface area of resulting powder.

Air-atomized grades of metal powder are produced by aspirating molten metal through a nozzle into a supersonic stream of air. The lower end of the nozzle dips into the bath of molten metal and its upper end terminates in a small orifice. When a jet of air is passed over the orifice of the nozzle it creates a suction effect, aspirating the liquid metal through the nozzle, into the airstream and disintegrating the molten stream into small, discrete particles.

When a stream of liquid is broken into individual droplets by a jet of gas, each droplet is initially flattened and elongated by the force of the gas stream. These droplets will rapidly return to a spherical shape in order to minimize surface area and surface energy. However, when some metals are atomized in air, a hard, dense oxide film forms on the liquid drop before it can relax into a sphere. This oxide shell causes the particle to solidify into the initial distorted shape of the droplet. Consequently, air-atomized aluminum powders, for example, particularly the smaller grades, are irregular in shape.

A specialized version of this process utilizes inert gases for the atomization process. Resulting powder is globular (approximately spheroidal) rather than spherical.

Other common methods of powder production include carbonyl decomposition, water atomization, hydrogen or carbon reduction of metallic oxides, and condensation of metal vapor such as the production of zinc dust. All these methods produce a distinctly three-dimensional product.

The qualitative description of particle shapes used herein follow the definition and vocabulary issued in standard 3252 of the International Standards Organization as reported in Powder Metallurgy, Principles and Applications of F. V. Lenel, Metal Powder Industries Federation, Princeton, N.J. (1980), under Particle Shape and Structure.

    ______________________________________                                         acicular     needle shaped                                                     angular      sharply edged or roughly polyhedral                               dendritic    of branched shape                                                 fibrous      having the appearance of regularly or                                          irregularly shaped threads                                        flaky        plate like                                                        granular     approximately equidimensional but of                                           irregular shape                                                   irregular    lacking any symmetry                                              nodular      of rounded irregular shape                                        spheroidal   roughly spherical                                                 ______________________________________                                    

These powders are illustrated by FIGS. 3-4, 3-5, 3-6, 3-7, 3-8, 3-9, 3-10, 3-11 and 3-12 in the above-referenced text and these Figures are included by reference.

All the above described particle shapes can be further described by average particle size, maximum and minimum particle size, particle size distribution, surface area, surface roughness, surface oxides, degree of agglomeration, etc.

To further distinguish among the described particles, a distinction can be made with respect to the three dimensions of the particles:

    ______________________________________                                         Particle Type     Primary Dimensions                                           ______________________________________                                         Acicular          1                                                            Angular           3                                                            Dendritic         2 or 3                                                       Fibrous           1, 2 or 3                                                    Flaky             2                                                            Granular          3                                                            Irregular         3                                                            Modular           3                                                            Spheroidal        3                                                            ______________________________________                                    

It can be seen that while all the described particles have three dimensions, in the case of needle-like particles, two of the dimensions are numerically small in comparison with the third dimension. Similarly, flaky particles have two primary dimensions, the third being generally extremely small.

For purposes of a group definition, all metal powders previously described-atomized, hydrogen reduced, carbonyl decomposed and all other essentially three primary dimension metallic powders are defined as three-dimensional powder.

In accordance with the invention, any powder which as defined above, has particles which have three-dimensional particles can be used, and these are used in conjunction with particles with two primary dimensions, typically flaky material, as will be described further below. Accordingly, the powders of the invention contain three-dimensional particles or where the particles are other than three primary dimension, they are in smaller proportion; the other component of the powder mixture is a particle with two primary dimensions, typically a flake.

In contrast to production of three-dimensional metal powders, metallic flake is produced by wet or dry ball milling processes. Aluminum flakes may be leafing or non-leafing in their wetting properties, while all metal flakes may have a variety of typical particle sizes and ranges. Flake pigments have two primary dimensions with a small thickness. Flakes have high aspect ratios; that is, the ratio of length or width to thickness is a large number. Particle sizes are not easily measured, but are estimated by measuring surface area per gram of pigment.

Paint manufacturers generally use flake pigments in coatings in preference over atomized powders. Flake pigments are excellent waterproof barriers to corrosion because of the lamellar orientation of the flakes throughout the film. In addition to barrier effects, water transmission (or vapor transmission) is decreased due to the path length necessary to pass through the coating and around the flake pigment particles. In addition to these properties, flake pigments reflect light providing for cooler surfaces that are bright and attractive.

While useful in conventional paint formulations, incorporation of flake pigments in chromate/phosphate binders causes a variety of problems. The problems relate to the water based inorganic nature of the liquid coating material and the curing mechanism of the coating. The coating cures and becomes water insoluble by a group of reactions that involve dehydration. Since flake pigments pack and overlap in a lamellar structure, water loss causes disruption of the film.

Consequently, flake-containing chromate/phosphate and chromate films made in accordance with the above referred to references are very thin with thicknesses so thin that thickness measurements are made by coating weight after curing rather than simple thickness measurements. See for instance, Baldi '046. To obtain a "thick" coating, it is necessary to perform multiple coating/curing application in which 8-10 application and cure cycles to 800° F. are employed to safely produce a coating 12-15 micrometers thick.

In work involving the invention, attempts to make a "thick" coating that could be applied in one or two coats illustrate another problem of incorporating flake pigments in chromate/phosphate coatings. Additions of flakes at levels high enough to achieve a reasonable film build were found to be impossible due to the increase in viscosity of the resulting coating, even at relatively low levels of added flake.

FIG. 1 shows how viscosity changes occur with increasing pigment level for six different aluminum powders; two are air atomized and four are flake powders. Table 1 lists the powders in FIG. 1. Each powder was dispersed in Binder A at various pigment concentrations beginning at 10 grams per 100 milliliters, then increasing the pigment concentration and evaluating the resulting viscosity. The binder material was made as follows:

    ______________________________________                                         Binder A                                                                       ______________________________________                                         3,200     ml         deionized water                                           1,525     gm         phosphoric acid 85%                                       350       gm         chromic acid                                              300       gm         magnesium oxide                                           ______________________________________                                    

This binder has the following physical properties:

    ______________________________________                                         pH          1.6                                                                Viscosity   30 cps (Brookfield LV, No. 2 spindle)                              Density     1.28 grams/cc                                                      Weight per solids                                                                          28.0                                                               ______________________________________                                    

In all cases, the flake pigments regardless of processing type (leafing or non-leafing) follow the same curve with very high viscosities at pigment concentrations of 30 grams per 100 milliliters of Binder A. The atomized type pigments, numbers 5 and 6 show markedly less viscosity increase even at triple the pigment concentration. Average particle size of the flake did not seem to affect viscosity for the flakes tested (compare pigments numbers 2 and 3).

FIG. 2 shows the same curves, but the "x" axis has been changed to be based on flake/atomized powder concentration per unit of water available from the binder to "wet" the powder.

Table 1 Data for Aluminum Powders

1. Reynolds 40XD leafing flake aluminum powder; 0.1% retained on a 325 mesh screen.

2. Reynolds 4-301 non-leafing flake aluminum powder; 4.0% retained on a 325 mesh screen.

3. Reynolds 4-501 non-leafing flake aluminum powder; 10% retained on a 325 mesh screen.

4. Reynolds LSA 904 non-leafing flake aluminum paste; 0.1% retained on a 325 mesh screen.

5. Valiment H-3 spherical aluminum powder; average particle size (FSSS) 3.4 μm.

6. Reynolds LSA 693 air atomized aluminum powder; average particle size (FSSS) 4.5 μm.

It can be seen from FIGS. 1 and 2 that high viscosities are quickly achieved when flake constituents are added to chromate/phosphate binders. However, coatings with viscosities in excess of several thousands cps often need special spray equipment and techniques. In addition, as noted previously, these coatings "cure" by water loss and densely packed coatings that consist of packed flakes are very difficult to cure. Blistering is expected.

In addition, flake components are difficult to "wet" or completely disperse in the aqueous medium. As previously noted, FIG. 2 is the same graph of six aluminum powders, but with the "x" axis relabeled to show the relationship of flake or atomized powder to the amount of water available to wet the particles. Flakes have high surface areas and are difficult to "wet" or disperse in chromate/phosphate binders because the surface tension of such acid solutions exceeds 70 dynes/cm. Surface tension reduction is required if an optimum dispersion of the flakes is to be achieved. Aluminum flake powders are especially difficult to work with due to an organic film that is deposited on the surface of each flake in the manufacturing process, resulting in a high resistance to wetting in an aqueous medium, and a high tendency to generate large amounts of foam to achieve wetting.

FIG. 3 shows the effect of a non-leafing aluminum flake (Reynolds 4-301) on the viscosity of a chromate/phosphate binder of the invention. At about 10 weight percent, the viscosity is almost 500 cps; at 20 weight percent it is almost 1,500 cps; at higher concentration, the viscosity very markedly increases.

It is very helpful under these conditions to add a surfactant to the chromate/phosphate binder. While there are many surfactants that might provide effective wetting of the flake pigment, a combination of special properties is necessary for use in these flake coating compositions. First, the useful surfacant must be stable in the acidic chromate/phosphate binder. Most organic materials are readily oxidized by chromate (dichromate) ions at the prevailing low pH. Second, the surfactant must have a low foaming tendency or ideally, have some defoamer character. When mixing flake compositions, flakes that have not been fully "wet" tend to get trapped in any foam layer that forms above the coating liquid. This actually stabilizes the foam network, making it even more difficult to wet out the flakes.

Nonionic surfactants of the ethoxylated alkyl phenol or aliphatic alcohol family such as those known under various trademarks as Igepal, Levelene, Neutronyx, Surfonic and Triton have been found useful. In particular Triton X-100 was found to have good wetting and stability. Nonionic tertiary glycols such as Surfynol 104 are also useful. Cationic secondary and tertiary amines of the polyoxy cocamine type exemplified by Armak Ethomeen C/20 and Emery 6601 were found to be especially useful in wetting out aluminum flakes that have an oleate or stearate film deposited on the surface. Also useful were quaternary amines such as Armak Ethoquad R/13-50. Additionally, anionic surfactants of the group that includes sodium heptadecyl sulfate, sodium tetradecyl sulfate and sodium 2-ethylhexyl sulfate are usable wetting agents.

In addition to application difficulties, flake pigmented chromate/phosphate coating have formability problems. Flake filled coatings bend poorly and are easily eroded because particles cannot effectively absorb impacts. Similar results are achieved with direct and reverse impact tests. Coating loss occurs making the coatings unsuitable for most applications.

Most metal filled chromate/phosphate coatings cited in the patent art utilize atomized aluminum powder as part or all of the pigment. Starting with the Allen '251 patent, coatings are described using 5-10 micrometer powder, that is, powder generally averaging 5-6 μm in diameter. These powders are air atomized and can be purchased from a variety of sources. The powder varies slightly since each manufacturing plant produces slightly different powder. While the powder may vary slightly in particle distribution, the particle shape is similar. The powder is granular in shape with rounded shapes varying in length and size, but with no sharp edges. Coatings utilizing this general type of aluminum pigment are frequently used as anti-corrosive coatings and coatings to resist elevated temperature. Such coatings can be made galvanically active, or sacrificial by burnishing, i.e., mechanically cold working the surface of the coating or by heat treatment at about 550° C. for one hour. In each case, the coating becomes electrically conductive possibly by a combination of physical processes and chemical interaction.

The general shape of the pigment allows for some packing of the aluminum particles. The packing of the particles generally produces coatings of excellent tensile bond strength; when coating failure occurs bonding values often exceed 8,000 psi (70 MPa). Although the bond strength values are high, interstitial voids are common and vapor/moisture penetration can easily occur. This problem is less apparent in coatings made in accordance with U.S. Pat. No. 4,592,958 where small spherical powder is used in similar chromate/phosphate coatings. Bond strengths are very high, packing is such that little moisture penetration occurs and corrosion is minimized.

Coatings utilizing both types of atomized powder are easily formed and have excellent erosion resistance against particle impact. After this invention was made, it is now thought that the improved properties appear to be due, in part, to the three-dimensional particle shape which allows impacts to be absorbed or transmitted from particle to particle within the chromate/phosphate matrix.

In some situations in which coatings are made in accordance with the teachings of these reference patents particularly when utilizing these coatings for dip/spin coating application, there remain areas on coated parts in which coverage is incomplete or quite variable in thickness. An example is a fine machine screw thread. Coating tends to build in the root of the thread and accumulate also at the crest of the thread. The line between these points can often be bare. This becomes an increasingly serious problem when surface preparation is minimal and consists only of alkaline cleaning or pickling. Such variation in coating thickness is related to the forces produced during the dip/spin process. Under high shear conditions, the coating viscosity decreases because of its thixotropic or pseudoplastic nature and the pigment, being of rounded shape, migrates to the roots or crests where it builds up. This effect can easily be seen in coatings using air atomized aluminum as pigment and also applications using inert gas atomized aluminum as pigment.

It has been found, in accordance with the invention, that by the addition of flake aluminum powder in addition to the atomized aluminum powder improved coverage results and coating performance improves. In particular, non-leafing flake aluminum powder has been shown to perform well in these compositions. In these applications, the flake pigment, being flat, does not move under the dip/spin applied stress resulting in more coating uniformity and no bare areas along the thread pitch.

It was expected that the addition of flake aluminum, particularly non-leafing flake aluminum, would have a detrimental effect on the coating composition by decreasing pigment packing density and consequently lower the corrosion resistance. However, in spite of the random distribution of the flakes amongst the three-dimensional (e.g. spheroidal) pigments, the coatings obtained in accordance with the invention were found to be more uniform in coating thickness and as a result show improved corrosion resistance because of elimination of bare areas. A series of these test coatings is described in Examples 12 through 16.

The unexpected improvement in applicability and resulting coating uniformity in dip/spin application is surprising in that disruption of particle packing effects was also expected. These disruptive effects do tend to appear as flake content of the coating composition increases, but are not a severe problem until flake content is dramatically increased.

In addition to the improved uniformity of the applied films, the films are often thinner per coat than if the coatings contained only three-dimensional powder. Films have erosion resistance similar to coatings that include only three-dimensional powder and have good formability.

Part of the improved corrosion resistance seen with coatings of the invention are a result of more complete coating coverage; however, additional corrosion protection seems to be related also to the lamellar structure of the flakes. This plate-like deposition of flakes among three-dimensional pigment particles increases the length of the path required for moisture or vapor to permeate the coating and reach the substrate.

Additionally, as shown in FIGS. 1 and 2, the increase in viscosity of flake-only chromate/phosphate compositions was shown. It is further seen in FIG. 3, that by mixing the flake and atomized spheroid pigments it is possible to produce controlled viscosity increases. A 10% flake addition will produce a 500 centipoise viscosity coating, etc. This allows for high/low or designed viscosities to be built into coating composition without having to use additives such as gums and other polymers.

It has been found that the concept is not only applicable to atomized aluminum powder/aluminum flake pigment mixtures, but other metallic pigments can be used in conjunction with a wide variety of flake materials described hereinafter.

In accordance with the invention, the coating composition of the invention comprises an acid binder which comprises phosphate ions and ions from the group of chromate and molybdate and pigments previously described and further described hereinafter.

The acid binder is an aqueous liquid having a pH less than about 4.0. While various organic solvents may be included to improve wetting of pigments and other physical properties, and to act as co-solvents, the binder is primarily water based.

The phosphate ion can be supplied from a variety of sources including phosphoric acid solutions and other materials such as phosphate salts of metals including monobasic, dibasic and tribasic salts. These salts can be used in conjunction with phosphoric acid to produce acid phosphate solutions. Other anhydrides or compounds that produce phosphate in solution can be used such as phosphorous pentoxide, metaphosphorous acid, phosphorous acid and hypophosphorous acid. Phosphorous acid and hypophosphorous acid produce phosphate ion in these binders by reaction with oxidizing agents such as hexavalent chromium in solution.

In addition to phosphate ion, the binders include hexavalent chromium in solution. While describing this form of chromium as chromate, it is to be understood that under acid conditions the ion is probably better described as a dichromate. The term "chromate" is used by convenience. The chromate in solution may be supplied by chromic acid or metal chromate or dichromate. Alternatively, molybdate ion can be used. The ion can be supplied from molybdenum trioxide or metallic molybdates.

In addition to phosphate and chromate or molybdate, metal ions may be dissolved in the binder. Divalent and trivalent ions are most suitable for inclusion in the compositions, but compositions containing no metal ions can be used as well as monovalent ions. Ratios of phosphate to chromate or molybdate in solution can be very wide. The addition of chromate especially produces a binder material that is amorphous when thermally cured and less brittle than a phosphate only binder. In general, molar ratios of PO₄ to CrO₄ will be between 1 and 4 to 1.

The binder portion as previously described is combined by high speed mixing or simple stirring with the pigment portion. As noted, the pigment portion comprises a mixture of pigments, one pigment being a flake pigment, the other generally being spherical or spheroid. The usable pigments for the invention include most metals and their alloys. Materials of special interest for the invention are:

    ______________________________________                                         steel powder (low alloy)                                                       stainless steel powder                                                         copper powder, copper alloy powder                                             cobalt powder                                                                  tin powder                                                                     tungsten powder                                                                molybdenum powder                                                              tantalum powder                                                                titanium and its alloys powders                                                zirconium and its alloys powders                                               transition metal carbide powders                                               beryllium powder                                                               ______________________________________                                    

Of particular interest are the following:

    ______________________________________                                         spherical nickel powder                                                        silver coated nickel powder                                                    spherical atomized aluminum powder                                             air atomized aluminum powder                                                   atomized aluminum alloy powder                                                 iron powder (hydrogen reduced from oxide)                                      silicon powder                                                                 silver powder                                                                  nickel and nickel based alloy powders                                          refractory metal powders                                                       ______________________________________                                    

A list of flake materials suitable for the invention includes:

    ______________________________________                                         aluminum powder, non-leafing flake                                             aluminum powder, leafing flake                                                 nickel flake - such as Novamet HCAl                                            nickel fine flake                                                              stainless steel flake                                                          steel flake                                                                    copper/nickel flake                                                            mica and chemically treated mica based pigments                                micaceous iron oxide                                                           ______________________________________                                    

Use of zinc flake, zinc dust, or magnesium metal powder is not contemplated by this invention due to high reactivity of the powders with the binder material.

Materials and techniques useful for the practice of the present invention include those disclosed in the following U.S. Pat. Nos.: 4,532,289; 4,537,632; 4,544,408; 4,548,646; 4,564,555; 4,592,958; 4,606,967; 4,610,929; 4,617,056; and the fixture for centrifugal dip/spin coating processes for coating parts disclosed in 4,489,670, all of which are incorporated herein by reference.

The relative concentration of pigments to binder solids can be highly variable and still produce coating compositions having good bond strength and improved corrosion resistance. It has been found that, for example, an aluminum filled coating may have a ratio of metal powder to binder solids of 2.5 to 1; the same volume of nickel pigment will produce a weight ratio of 7.8 to 1 in relation to binder solids.

The following examples are illustrative of the invention and are not intended to be limiting. It is evident to one skilled in the art that the ingredients of the various compositions illustrated (e.g., their relative proportions and amounts), as well as other variables and parameters can be modified while being within the scope and contemplation of the invention.

EXAMPLE 1 Binder I

    ______________________________________                                         731        ml        Deionized water                                           187.25     ml        85% phosphoric acid                                       103.63     gm        Chromic acid                                              21.1       gm        Magnesium oxide                                           66         gm        Magnesium carbonate                                       ______________________________________                                    

Coating composition:

    ______________________________________                                         272     ml      Binder I                                                       106     ml      Deionized water                                                0.2     ml      Potassium Perfluoroalkyl Sulfonate,                                            FC-95 (3M Company)                                             288     gm                                                                                      ##STR1##                                                                      3.4 μm                                                      41      gm      Non-leafing aluminum flake powder,                                             Reynolds 4-301                                                 ______________________________________                                    

The coating was mixed by blending the ingredients using an Osterizer blender. The aluminum flake contents is 12. 5% of the total pigment. Sample Tek fasteners were coated with two coats of the composition by the dip/spin method and cured at 650° F. These fasteners exhibited superior salt spray corrosion resistance when alumina grit burnished to make the coating electrically conductive.

EXAMPLE 2 Binder II

    ______________________________________                                         805       ml         Deionized water                                           220       ml         85% phosphoric acid                                       92        gm         Chromic acid                                              60.5      gm         Magnesium oxide                                           ______________________________________                                    

A coating composition was prepared by blending the following ingredients:

    ______________________________________                                         200    ml      Binder II                                                       5      ml      Surfactant, Niaproof 08 (Niacet                                                Corporation)                                                    162    gm      Aluminum powder, spherical, average                                            equivalent spherical diameter 3.4 μm                         18     gm      Aluminum flake powder, Reynolds 40XD                            ______________________________________                                    

The coating composition was applied by the dip/spin method to several types of steel fasteners. When sectioned and examined using a metallograph, the coating appeared to be uniform in thickness on both the threads and the shank of the fastener.

EXAMPLE 3

The following composition was prepared by mixing the ingredients in order and stirring for 15 minutes.

    ______________________________________                                         200     ml      Binder II                                                      50      ml      Deionized water                                                0.2     gm      Surfactant, Surfynol 104 (Air Products                                         & Chemicals Inc.)                                              175     gm      Aluminum powder, air atomized, average                                         particle size 16 μm (FSSS)                                  25      gm      Non-leafing aluminum flake powder,                                             Reynolds 4-591                                                 ______________________________________                                    

Mild steel test panels were coated by spray applying two coats of the composition and curing each coat at 340° C. for about 30 minutes.

To evaluate the performance of the coating, sample steel panels utilizing the Example 3 composition were compared with steel panels coated with a similar composition not including the flake component. Testing was done by immersion in a 5% sodium chloride solution which was modified by the addition of 1 weight percent of 30 percent hydrogen peroxide. This test method is known as the Machu test (Machu W, Schiffmann L and Hudson JC "Uber ein neues Kurzzeit-Korroisonprufoverfahren fur phosphatierte bzw, chromatierte, mit Anstrichen versehene Verzinkte Eisenbleche". Metalloberflaeche, Vol 9, 1968 pg. 274-277). After 16 hours of immersion, panels coated with the composition showed 50% less corrosion than similar specimens without the flake addition.

EXAMPLE 4 Binder IV

    ______________________________________                                         852.5     ml         Deionized water                                           22.3      gm         Magnesium oxide                                           54        gm         Chromic acid                                              94.6      ml         85% phosphoric acid                                       ______________________________________                                    

Coating composition:

    ______________________________________                                         Part 1:                                                                        ______________________________________                                         170    ml      Binder IV                                                        54     gm                                                                                     ##STR2##                                                       6      gm      Non-leafing aluminum flake powder                                              Reynolds 4-591                                                  1      ml      Polyoxyethylene cocamine surfactant,                                           Ethomeen C/20 (Armak Chemicals)                                 ______________________________________                                    

Mix by high speed dispersation.

    ______________________________________                                         Part 2:                                                                        ______________________________________                                         170        ml         Binder IV                                                20.5       gm         Strontium chromate                                       ______________________________________                                    

Ball mill 4 hours.

The coating composition was prepared by mixing equal volumes of PART 1 and PART 2. When spray applied to cold rolled steel test panels and cured at 600° F., the coating showed outstanding performance when tested in a Copper Accelerated Acetic Acid Salt Spray Test (CASS) per ASTM B368. Use of leachable chromate additions to chromate/phosphate based coating compositions is described in pending patent application No. 777,228 of Sept. 19, 1985 which is incorporated herein by reference.

EXAMPLE 5 Binder V

    ______________________________________                                         360       gm         85% phosphoric acid                                       89.3      gm         Chromic acid                                              70.4      gm         Magnesium oxide                                           Vol. to 1 Liter  Deionized water                                               ______________________________________                                    

Composition:

    ______________________________________                                         100    ml      Binder V                                                        150    gm      Nickel powder, Alcan #123                                       16.7   gm      Leafing nickel flake powder, Novamet,                                          INCO Selectric Surfaces Inc.                                    2.66   gm      Fumed silica, Cab-O-Sil M5                                      0.1    ml      Nonionic surfactant, Triton X-100, Rohm                                        & Haas                                                          ______________________________________                                    

The coating was blended for 10 minutes in a high speed mixer.

The coating was applied to stainless steel automotive bushings where it acted as a high temperature anti-fretting coating. The flake component acts to promote uniform coating thickness and the atomized powder portion is compressible under the clamping load.

EXAMPLE 6

A coating composition which uses flake and sphere powders in a dip/spin composition with high viscosity as taught in U.S. Pat. No. 4,548,646 to Mosser and Fabiny was made according to the following formula:

Binder VI

    ______________________________________                                         500       ml         Deionized water                                           27.9      gm         Chromic acid                                              85        gm         85% phosphoric acid                                       18.73     gm         Magnesium carbonate                                       ______________________________________                                    

Coating composition:

    ______________________________________                                         504     ml    Binder VI                                                         230.36  gm                                                                                   ##STR3##                                                        32.91   gm    Non-leafing aluminum flake powder,                                             Reynolds 4-301                                                   0.25    gm    Potassium perfluoroalkyl sulfonate,                                            FC-95, 3M Company                                                ______________________________________                                    

Blend 15 minutes using a high speed mixer.

    ______________________________________                                         9.4     gm      Fumed silica Cab-O-Sil M-5                                     1.2     ml      Nonionic surfactant, 10% solution of                                           Triton X-100                                                   Stir 10 minutes.                                                               ______________________________________                                    

The coating was applied to degreased, grit blasted self tapping fasteners by the dip/spin method. When tested in 5% salt spray, the coating exhibited superior corrosion resistance to similar coatings that did not contain the aluminum flake/sphere combination, i.e., contained only the spherical powder.

EXAMPLE 7

    ______________________________________                                         200    ml      Binder II                                                       4      ml      Anionic surfactant, Emphos PS 21A,                                             Witco Chemical Company                                          4      ml      Diacetone alcohol                                               24     gm      Leafing aluminum flake powder, Reynolds                                        5XD 90% minus 325 mesh                                           16     gm                                                                                     ##STR4##                                                       ______________________________________                                    

The coating composition was spray applied to test panels which had been previously coated with a composition similar to Example 7 of U.S. Pat. No. 3,248,251 to Allen. All of the coatings were cured at 650° F. After 48 hours immersion in a Machu test, only a few very small pinholes of red rust were observed on the surface of the coating.

EXAMPLE 8 Binder VIII

    ______________________________________                                         328      ml       Deionized water                                              78.35    ml       85% phosphoric                                               23.68    gm       Chromic acid                                                 25.75    gm       Magnesium oxide                                              4.5      gm       Aluminum hydroxide, dried gel                                ______________________________________                                    

Coating Composition:

    ______________________________________                                         200    ml       Binder VIII                                                    100    gm       Atomized aluminum powder average                                               particle size (FSSS) 16 μm                                  45     gm       Spherical aluminum powder average                                              particle size (FSSS) 3.5 μm                                 10.14  gm       Non-leafing aluminum flake powder                              0.12   gm       Nonionic surfactant, Surfynol 104, Air                                         Products & Chemicals Company                                   ______________________________________                                    

This composition represents the addition of a non-leafing aluminum flake powder to a thick coating composition of the type described in U.S. Pat. No. 4,617,056 to Mosser and McMordie. When tested in the Machu test on steel panels, this composition showed excellent resistance to moisture permeation.

EXAMPLE 9 Binder IX

    ______________________________________                                         3,200     ml         Deionized water                                           1,525     gm         Phosphoric acid, 85%                                      350       gm         Chromic acid                                              300       gm         Magnesium oxide                                           ______________________________________                                    

Composition 9A

    ______________________________________                                         385    gm      Binder IX                                                        430    gm                                                                                     ##STR5##                                                        110    gm                                                                                     ##STR6##                                                       ______________________________________                                    

Mix using high shear mixer for 15 minutes.

Composition 9B

    ______________________________________                                         385    gm      Binder IX                                                        430    gm                                                                                     ##STR7##                                                        110    gm                                                                                     ##STR8##                                                       1      ml      Anionic surfactant, Niproof 08                                  25     gm      Non-leafing aluminum flake powder,                                             Reynolds 4-301                                                  ______________________________________                                    

Composition 9A represents a coating similar to Example 7 of U.S. Pat. No. 4,617,056 to Mosser and McMordie except that the thickening ingredients were excluded. Composition 9B is the same coating with the addition of a non-leafing aluminum flake powder and a suitable surfactant. Sixteen hours after mixing the viscosity of the 9A, composition was 420 cps while 9B was 2,200 cps, using a Brookfield LV viscometer and a #3 spindle at 30 rpm. On standing for two weeks, the 9A composition had settled with a densely packed layer of pigment on the bottom of the container. The 9B composition also had a settled pigment layer. However, it could be redispersed very easily.

EXAMPLE 10

    ______________________________________                                         295    ml      Binder IX                                                        283    gm                                                                                     ##STR9##                                                       ______________________________________                                    

The ingredients were mixed for 15 minutes using high speed dispersation. The viscosity of the resulting composition was 830 cps using a Brookfield Helipath Viscometer with a T-C spindle.

To the above composition the following ingredients were admixed:

    ______________________________________                                         75    ml      Binder IX                                                        75    gm      -325 mesh Mica powder, Martin Marietta                                         325 HK                                                           ______________________________________                                    

The viscosity of the resulting composition was 1,328 cps. This composition represents a coating that is useful in providing a corrosion resistant basecoat on steel parts by either spray application or dip/spin techniques.

EXAMPLE 11

    ______________________________________                                         380    ml      Binder IX                                                        280    gm                                                                                     ##STR10##                                                      190    gm      Non-leafing nickel flake powder,                                               Novamet fine grade                                              0.5    ml      Anionic surfactant, Niaproof 08                                 ______________________________________                                    

This composition was mixed using a high shear mechanical mixer for 15 minutes, then poured through a 325 mesh screen.

This composition was used as a high temperature corrosion resistance coating on steel in which the presence of the nickel flake served as an "in situ" barrier to diffusion of the aluminum pigment into the steel substrate at temperatures exceeding 1,200° F.

The composition was applied to alloy steel turbine cases which were exposed to an operating temperature of 600° C. The coating functioned very satisfactorily.

EXAMPLE 12 Binder XII

    ______________________________________                                         373          gm       85% Phosphoric acid                                      92           gm       Chromic acid                                             75           gm       Magnesium oxide                                          Vol. to 2,000                                                                               ml       Deionized water                                          ______________________________________                                    

Coating composition:

    ______________________________________                                         200     ml      Binder XII                                                     160     gm      Aluminum powder, spherical, average                                            equivalent spherical diameter 3.4 μm                        ______________________________________                                    

The above mixture was processed under high shear conditions for 5 minutes and spray applied to aluminum oxide blasted steel panels, dried at 80° C. for 15 minutes then cured at 300° C. for 30 minutes.

EXAMPLE 13

    ______________________________________                                         200    ml     Binder XdII                                                      0.10   gm     Surfactant-potassium perfluoroalkyl                                            sulfonate                                                        150    gm     Aluminum powder, spherical as Example 12                         10     gm     Aluminum flake, non-leafing, Reynolds                                          4-301                                                            ______________________________________                                    

The coating was processed as Example 12, then the flake component was added and mixed by slow stirring. It was applied and cured as Example 12.

EXAMPLE 14

As Example 13 but with 20 grams of flake powder and 140 grams of spherical powder for a total of 12.5% flake based on metal pigment.

EXAMPLE 15

As Example 13 but with 30 grams of flake powder and 130 grams of spherical powder for a total of 18.8% flake based on metal pigment.

EXAMPLE 16

As Example 13 is repeated but with 40 grams of flake powder and 120 grams of spherical powder for a total of 25% flake based on metal pigment. 

We claim:
 1. A phosphate coating composition of improved uniform coating thickness, impact resistance, bond strength and corrosion resistance, which coating comprises (a) an aqueous acid binder solution which comprises phosphate ions and ions selected from the group consisting of chromate and molybdate ions and (b) a mixture which contains powder of spherical particles, and flakes in a random distribution amongst the particles, wherein said flakes constitute at least about one percent by weight of the mixture, with the proviso that neither the powder nor the flake is zinc or magnesium and that the phosphate ions are present in major proportion and in excess of the ions selected from the group consisting of chromate and molybdate.
 2. The phosphate coating of claim 1 wherein the powder of spherical particles and the flakes are metals or alloys thereof.
 3. The phosphate coating of claim 1 wherein the weight ratio of phosphate to chromate is greater than from about 2 to
 1. 4. The phosphate coating of claim 3 wherein the weight ratio of phosphate to chromate is greater than from about 2 to 1 and less than about 5 to
 1. 5. The phosphate coating of claim 1 wherein the weight ratio of phosphate to chromate is about 4 to
 1. 6. The phosphate coating of claim 2 wherein the mixture of spherical particles and flakes is in a ratio in the range of about 1 to about 65 percent by weight of the mixture.
 7. The phosphate coating of claim 6 wherein the ratio is in the range of about 5 to about 65 percent by weight of the mixture.
 8. The phosphate coating of claim 7 wherein the ratio is in the range of about 5 to about 25 percent by weight of the mixture.
 9. The phosphate coating of claim 3 wherein the flake and powder mixture is dispersed in the aqueous binder solution.
 10. The phosphate coating of claim 1 wherein in the binder solution, the ions selected from the group consisting of chromate and molybdate, are molybdate ions only.
 11. The phosphate coating of claim 3 wherein the composition is thixotropic.
 12. The phosphate coating of claim 3 wherein the composition has a viscosity which is not in excess of about 4,000 cps.
 13. The phosphate coating of claim 3 wherein the weight percent of flake to total powder--flake mixture is between about 1 to about 30 percent.
 14. The phosphate coating of claim 13 wherein the weight percent of flake to total powder--flake mixture is between about 10 to about 30 percent.
 15. The phosphate coating of claim 3 wherein the liquid binder in addition to the chromate ions, also contains leachable chromate.
 16. The phosphate coating of claim 3 wherein the powder particles are spheroidal or spherical.
 17. The phosphate coating of claim 3 wherein the metal flake is aluminum.
 18. The phosphate coating of claim 3 wherein the metals are both aluminum or alloys thereof.
 19. The phosphate coating of claim 3 wherein the metal spherical powder particles are selected from the group consisting of aluminum, nickel, silver, reduced iron, a refractory metal and the respective alloys thereof.
 20. The phosphate coating of claim 19 wherein the metal spherical powder particles are aluminum or alloys thereof.
 21. The phosphate coating of claim 2 wherein the metal flakes are selected from the group consisting of aluminum, nickel, iron and the alloys thereof and copper-nickel alloy.
 22. The phosphate coating of claim 2 wherein the powder spherical particles are metal particles which are selected from the group consisting of aluminum, nickel, silver, reduced iron, a refractory metal and the alloys thereof and wherein the flakes are metal flakes selected from the group consisting of aluminum, nickel, iron and the alloys thereof and copper-nickel.
 23. The phosphate coating of claim 17 wherein the flake is a non-leafing aluminum flake.
 24. The phosphate coating of claim 20 wherein the metal powder is a bimodal powder.
 25. The phosphate coating of claim 20 wherein the metal powder is a unimodal powder.
 26. The phosphate coating of claim 20 wherein the aluminum powder includes air-atomized aluminum.
 27. The phosphate coating of claim 1 wherein the aqueous binder solution includes a surface-tension lowering agent.
 28. The phosphate coating of claim 27 wherein the surface-tension lowering agent is a wetting agent selected from the group of non-ionics and anionics.
 29. The phosphate coating of claim 17 wherein the flake is Reynolds 4-301 aluminum flake. 